Tom Watson is the man who likened James Murdoch to a mafia boss last year, so when the Labour MP's book about the phone hacking scandal – jointly authored with Martin Hickman – was launched there was likely to be rhetorical flourish or two. In fact Watson worked to tone down the language – up to a point anyway.

"We conclude," he said at a Westminster press conference packed with British and foreign journalists, "that the web of influence which News Corporation spun in Britain, which effectively bent politicians, police, and many others in public life to its will, amounted to a shadow state."

It was not, when asked later, that he regretted the mafia accusation, just that he wanted to be more judicious; although anybody fearing that the political attack dog had gone soft on Britain's ruling media family will be wrong. Or as Watson put it: "Like the newspaper they have closed, the Murdochs have now become toxic" – a statement that in reality is not yet true, but might become so depending on how charges, trials or otherwise play out in the coming months.

Dial M for Murdoch has few new revelations within its pages. The fact that the News of the World targeted Watson and other MPs on the culture, media and sport select committee to see "who was gay, who had affairs" in 2009 – a subject not obviously of immediate interest to Sunday buyers of the red top – had some edge.

That, though, is because of the ongoing select committee enquiry into whether News Corporation misled parliament over the extent of its knowledge about phone hacking. It was known that members of the committee had already been followed by "Silent Shadow" Derek Webb on the orders of the tabloid, and its lawyer Tom Crone.

Yet, to reduce the book to the amount of revelation contained within is to miss the point. For a start, Dial M for Murdoch is a pacy, readable account of the hacking saga – albeit one that gives Watson something of a leading role.

There is little fresh insight into the thinking inside News Corporation as its walls tumbled down, although this is a famously close company, and Watson was probably the last person that executives past and present would be prepared to co-operate with. Instead, the title's force comes partly from the fact that it is an attempt to sum up all that its writers believe is wrong about Murdoch's News Corporation, in an attempt to shift middle opinion against the company.

That the book launch was packed was testimony to something else too. Consider the simplicity of the wider narrative: here, after all, is the parliamentarian who in the manner of Erin Brockovich stood up to the world's most powerful media mogul. Not only that, but Watson's persistent assault on News Corporation has catapulted him from the status of washed-up backbencher to deputy chair of the Labour party in two heady years. Taking on the Murdochs used to be a sure route to career destruction at Westminster, now it has become a means to achieve power, at least at the heart of the Labour opposition.

The rhetoric also reveals that the task, from the authors' perspective, is incomplete. "This is a story about corruption by power," the final paragraph begins, which ran "from the criminal underground to the headquarters of London's police force, from the decks of yachts in the Mediterranean to farmhouses in the Cotswolds and the deep carpeted rooms of Downing Street." News Corp has been "publicly humbled" but also is only "partially dismantled"; indeed for all that has happened the company is "shaken and ostensibly apologetic, but it is still there and Rupert Murdoch is in charge".

Both authors will have been aware for some time that Rupert Murdoch appears at the Leveson inquiry next Wednesday. This time Tom Watson has no opportunity to ask the questions. So over 317 pages, it is an attempt to set the agenda before the 81-year-old media mogul fills the screens; in that Watson and Hickman have already had some success.